1. FIELD OF THE INVENTION
The present invention relates to a power semiconductor switching device. More particularly, it relates to an improvement in a gate controlled power semiconductor switching device.
2. DESCRIPTION OF THE PRIOR ART
FIG. 1 is a schematic sectional view showing the conventional gate turn-off thyristor device; and FIG. 2 is an equivalent circuit diagram of the thyristor device. At turn-off time, the load current is mainly passed through the highest resistance portion (5 n and 6 n in the equivalent circuit of the FIG. 2) in the central portion of the cathode-emitter away from the gate electrode because the transverse sheet resistance R.sub.gk of the gate layer P.sub.B between the cathode-emitter N.sub.E and the base layer N.sub.B is not negligible whereby the turn-off operation of the portion is difficult. Turn-off switching energy as "I (current).times.V (voltage).times.t (time)" in turn-off transient time is absorbed in the portion. As a result, a hot spot can easily result in the portion which is finally turned off.
In distinction from the transistor device, the thyristor has a pattern separating the contact surface by dividing the cathode-emitter N.sub.E into fine fragments in order to improve the turn-off characteristic whereby only certain parts of the cathode-emitters are turned on but the other parts remain without turn-on during the turn-off operation. In order to prevent the partial turn-on of the cathode-emitter from causing the total turning-on, an extremely large turn-on gate current [a turn-on gate current having low current amplification factor (less than 10 for Hfe) similar to a high voltage power transistor] is required.
In the application to an apparatus causing irregular rising time of forward load current (such as an inverter or a reversing driver DC servo motor), it is necessary to maintain the total conductive condition to expect rising of the load current whereby the forward gate current for maintaining the total conductive condition is as high as the base current of a power transistor.
The turn-off current gain is practically only about 3 to 5 whereby a reverse bias pulse generator is quite complicated.
Switching energy is locally concentrated at the turn-off time whereby allowable absorbing ability is low for the device as a whole. It is, therefore, necessary to provide a reapplied voltage dv/dt controlling circuit [a capacitor (1001), a diode (1002), a discharging resistor (1003) etc.] as shown by dotted line in FIG. 1. The capacitor (1001) corresponds to a commutation capacitor in a gate assisted turn-off thyristor device (U.S. Pat. No. 4,107,551) and a loss in the resistor (1003) is substantially equal to a loss of a thyristor force commutation circuit. Furthermore because, a silicon material is often required in comparison with a thyristor device, the process is complicated, economical and an effect can not be sufficiently attained, whereby many applications to main industrial apparatuses such as an uninterrupt power source, a chopper for cars, and variable frequency inverters operable near a commercial frequency, which have real demand, are prevented and the usage is limitted to only application to special devices or it has only an advantage as technological publications.